Phospholipid vesicles are being used as models to study the phenomenon of membrane fusion. An experimental project is proposed to examine the conditions and to understand the physical mechanism of fusion of acidic phospholipid vesicles induced by divalent ions. Preliminary work in this laboratory has demonstrated a direct correlation between vesicle fusion and the bilayer phase transition in the presence of divalent ions. Systematic measurements will be made on the kinetics of fusion of different acidic phospholipid vesicles as a function of temperature and divalent ion concentration. The experimental approach is a novel combination of dynamic light scattering and stopped-flow rapid-mixing turbidity techniques to measure fusion kinetics and to differentiate between fusion and aggregation rapid and non-perturbatively. The results will be analyzed in conjunction with differential scanning calorimetric data to determine whether the correlation is due to critical fluctuations near the membrane phase transition or to divalent ion asymmetry inside and outside the vesicles. The relation between transition enthalpy and the fusion anomaly will also be studied. The long-term objective of the project is to increase our understanding of the role of divalent ions in regulating many membrane fusion processes in cellular and subcellar activities.